1. Field of the Invention
The present invention relates to zoom lenses, in particular, compact standard zoom lenses with high zoom ratios.
2. Description of Related Art
In recent years, a zoom lens with a zoom ratio of 2.5 that includes the standard field angle (hereafter standard zoom lens) having, in order from the object side, a lens unit with negative refractive power and a lens unit with positive refraction power has essentially replaced the standard fixed focal length lens (a lens with about 50 mm focal length on a 35 mm camera) as the standard camera lens.
Because a standard zoom lens as described above is mounted on the camera body as the regularly used lens, miniaturizing the zoom lens becomes as important as high performance and inexpensive manufacture.
Although a zoom lens with two lens units with negative and positive refractive powers as explained above is common, other arrangements have been proposed.
For example, a compact two unit zoom lens having a total of seven lenses with the first lens unit having negative, negative and positive refractive power lenses, and the second lens unit having positive, positive, negative, and positive refractive power lenses is proposed in Japanese Laid-Open Patent Publication Sho 62-94812.
Moreover, in Japanese Laid-Open Patent Publication Hei 5-249376 of the present inventor, a two unit zoom lens with the first lens unit having only a negative lens and a positive lens is proposed.
A zoom lens disclosed in Japanese Laid-Open Patent Publication Sho 62-94812 above is compact, consisting of a small number of lenses. However, the zoom ratio is small at 2.2 to 2.3, and there is a need to increase the zoom ratio in order to develop standard zoom lenses of the future.
The zoom lens disclosed in Japanese Laid-Open Patent Publication Hei 4-114115 has a higher zoom ratio of 2.7. However, with an increase in the zoom ratio, the position of an aspheric lens surface is moved from the object side to the image side and an aspheric lens surface is used in the lens closest to the image side in the first lens unit. As a result, the degrees of freedom for correction of coma at the telephoto side and spherical aberration that increases with increased zoom ratio are increased. On the other hand, compared to a zoom lens using an aspheric surface for the object side lens in the first lens unit, the image correction performance for coma at the maximum wide-angle state, astigmatism, and distortion are decreased, resulting in deterioration of imaging performance at the maximum wide-angle state.
As explained above, Japanese Laid-Open Patent Publication Hei 5-249376 discloses a zoom lens with the number of lenses in the first lens unit being reduced by one lens to a negative lens and a positive lens. The zoom lens disclosed in Japanese Laid-Open Patent Publication Hei 5-249376 realizes a high zoom ratio and high performance by minimizing the length of the first lens unit. However, by making the first lens unit a two lens structure, the refractive power of the negative lens in the first lens unit became relatively strong, tending to cause the aspheric amount (the difference between a paraxial reference spherical lens surface and the aspheric lens surface in the direction of the optical axis) to become relatively large.
Hence, it is difficult to manufacture the above-stated negative lens using current manufacturing methods such as producing a compound type aspheric lens with the glass direct press method. Thus, not only do production costs rise, but also with the compound type aspheric lens, it is difficult to accept increased aberrations caused by moisture absorption and temperature changes in the synthetic resin lens components, even if production is possible.